I had already sent some coins to my seemingly invalid p2sh/segwit addresses - are these coins now entirely lost?
It's lost if you send it to an address that you don't fully control.
How did you import it to Electrum?
If you imported the private key to Electrum in a wrong way it would gives you a different address usually legacy address.
If I were you try adding the script type before importing the private key check the guide below as sample
p2wpkh: bc1......
p2wpkh-p2sh: 3.......
p2pkh: 1....